Selected Abstracts

CHEMINFORM, Issue 51 2009Huaqiao Tan
Abstract
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals.
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[source]

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 6 2000T. Ito
ABSTRACT
Photophysical characteristics of N-substituted C5,C5,-linked dihydrothymine dimers (1a,b[meso], meso compounds of [5R,5,S]-bi-5,6-dihydrothymines; 1a,b[rac], racemic compounds of [5R,5,R]-bi-5,6-dihydrothymines and [5S,5,S]-bi-5,6-dihydrothymines) in aqueous solution with varying contents of less-polar aprotic solvent such as tetrahydrofuran or dioxane have been investigated by UV-absorption, and steady-state and time-resolved fluorescence spectroscopies.
Among the C5,C5,-linked dimers, (5R,5,S)-bi-5,6-dihydro-1-methylthymine (1a[meso]) showed a redshifted weak UV-absorption band at 270,350 nm and excimer fluorescence emission at ,max= 370 nm with a quantum yield (,F) of ,0.1 in phosphate buffer (pH < 10) at 293 K. Racemic compound of 5,6-dihydro-1-methylthymine dimer (1a[rac]), meso and racemic compounds of 5,6-dihydro-1,3-dimethylthymine dimers (1b[meso] and 1b[rac]) in phosphate buffer were nonfluorescent under similar conditions.
The UV-absorption and fluorescence spectral characteristics of 1a[meso] in aqueous solution were interpreted in terms of intramolecular stacking interactions between the dihydropyrimidine chromophores leading to a preferential "closed-shell" conformation both in the ground state and the excited singlet state.
In basic solutions at pH > pKa= 11.7, the fluorescence quantum yield of 1a[meso] decreased due to a dominant "open-shell" conformation resulting from the electrostatic repulsion between the deprotonated dihydrothymine chromophores of 1a[meso] in a dianion form.
[source]

A list of organic kryptoracemates

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2010László Fábián
A list of 181 organic kryptoracemates has been compiled.
This class of crystallographic oddities is made up of racemic compounds (i.e. pairs of resolvable enantiomers) that happen to crystallize in Sohnke space groups (i.e. groups that include only proper symmetry operations).
Most (151) of the 181 structures could have crystallized as ordered structures in non-Sohnke groups.
The remaining 30 structures do not fully meet this criterion but would have been classified as kryptoracemates by previous authors.
Examples were found and checked with the aid of available software for searching the Cambridge Structural Database, for generating and comparing InChI strings, and for validating crystal structures.
The pairs of enantiomers in the true kryptoracemates usually have very similar conformations; often the match is near-perfect.
There is a pseudosymmetric relationship of the enantiomers in about 60% of the kryptoracemate structures, but the deviations from inversion or glide symmetry are usually quite easy to spot.
Kryptoracemates were found to account for 0.1% of all organic structures containing either a racemic compound, a meso molecule, or some other achiral molecule.
The centroid of a pair of enantiomers is more likely (99.9% versus 99% probability) to be located on an inversion center than is the centroid of a potentially centrosymmetric molecule.
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S,S -1,2-Dicyclohexylethane-1,2-diol and its racemic compound: a striking exception to Wallach's rule

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2006Brian O. Patrick
The structures of enantiopure S,S -1,2-dicyclohexylethane-1,2-diol and its racemic compound (rac - S,S -1,2-dicyclohexyl­ethane-1,2-diol) have been determined at 295 and 173,K.
The crystals of the enantiopure material are more than 4% denser than the crystals of the racemic compound, but the melting points indicate that the crystals of the less dense racemic compound are considerably more stable than those of the racemic conglomerate.
This apparent exception to the correlation of crystal density and melting point is explained.
The enantiopure crystals have four molecules in the asymmetric unit (Z, = 4).
Two of the molecules have the conformation observed for the one independent molecule of the racemic compound and two have a higher energy conformation; the overall P21 structure is a perturbed version of a P212121 structure with Z, = 2.
The enantiopure and racemic crystals have the same hydrogen-bonding motif, but the motif in the former appears to be significantly strained.
A reason why crystals of enantiopure material might be systematically less dense than crystals of its racemic compound and to be more likely to have Z, > 1 is suggested.
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ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2003Henning Osholm Sørensen
The structural and thermodynamic backgrounds for the crystallization behaviour of racemates have been investigated using 2-phenoxypropionic acid (PPA) as an example.
The racemate of PPA behaves normally and forms a racemic compound that has a higher melting point and is denser than the enantiomer.
Low-temperature crystal structures of the pure enantiomer, the enantiomer cocrystallized with n -alkanes and the racemic acid showed that hydrogen-bonded dimers that form over crystallographic symmetry elements exist in all but the structure of the pure enantiomer.
A database search for optically pure chiral mono-carboxylic acids revealed that the hydrogen-bonded cyclic dimer is the most prevalent hydrogen-bond motif in chiral mono-carboxylic acids.
The conformation of PPA depends on the hydrogen-bond motif; the antiplanar conformation relative to the ether group is associated with a catemer hydrogen-bonding motif, whereas the more abundant synplanar conformation is found in crystals that contain cyclic dimers.
Other intermolecular interactions that involve the substituent of the carboxylic group were identified in the crystals that contain the cyclic dimer.
This result shows how important the nature of the substituent is for the crystal packing.
The differences in crystal packing have been related to differences in melting enthalpy and entropy between the racemic and enantiomeric acids.
In a comparison with the equivalent 2-(4-chlorophenoxy)-propionic acids, the differences between the crystal structures of the chloro and the unsubstituted acid have been identified and related to thermodynamic data.
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Potential of different techniques of preferential crystallization for enantioseparation of racemic compound forming systems

ELECTROPHORESIS, Issue 22 2009Jurim Gwon
Abstract
Cellulose dimethylphenylcarbamate (CDMPC)-immobilized zirconia (CDMPCZ) was used as a chiral stationary phase for enantioseparation of a set of nine racemic compounds in reversed-phase CEC.
Influences of the type and composition of organic modifier and the applied voltage on enantioseparation were examined.
Separation data on CDMPCZ were also compared with those on CDMPC-immobilized silica (CDMPCS).
Enantiomers of the analytes investigated are well separated in ACN/phosphate buffer mobile phases.
Better enantioselectivity and resolution were obtained with ACN than MeOH as the organic modifier.
Retention was longer but better enantioselectivity and resolution were obtained on CDMPCZ than CDMPCS.
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ELECTROPHORESIS, Issue 19 2008Bo Lin
Abstract
Enantiomer separations were performed on three ,-cyclodextrin-based chiral stationary phases (CSP) containing the pernaphthylcarbamoylated ,-cyclodextrin (CSP 1), peracetylated ,-cyclodextrin (CSP 2) and permethylated ,-cyclodextrin (CSP 3) as chiral selectors by capillary liquid chromatography and pressure-assisted capillary electrochromatography in this study.
Triethylammonium acetate/MeOH or phosphate buffer/MeOH was used as the mobile phase.
The experimental factors affecting chiral separations have been examined for each CSP, including pH of the buffers, methanol content and applied voltage.
Under optimal separation conditions, a number of racemic compounds were resolved into their enantiomers on three cyclodextrin-based CSP.
A comparative study on the performance of three CSP revealed the presence of carbonyl functional groups as well as aromatic rings in the cyclodextrin derivatives, enhanced the interaction between the analytes and CSP, and thus improved enantioselectivity of the CSP.
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HELVETICA CHIMICA ACTA, Issue 2 2007Romain Siegrist
Abstract
In this paper, we describe the synthesis and biological evaluation of highly substituted perhydropyrrolizines that inhibit the peptidyl-prolyl cis/trans isomerase (PPIase) Pin1, an oncogenic target.
The enzyme selectively catalyzes the cis/trans isomerization of peptide bonds between a phosphorylated serine or threonine, and proline, thereby inducing a conformational change.
Such structural modifications play an important role in many cellular events, such as cell-cycle progression, transcriptional regulation, RNA processing, as well as cell proliferation and differentiation.
Based on computer modeling (Fig.,2), the new perhydropyrrolizinone derivatives (,)- 1a,b, decorated with two substituents, were selected and synthesized (Schemes,1,3).
While enzymatic assays showed no biological activity, 15N,1H-HSQC-NMR spectroscopy revealed that (,)- 1a,b bind to the WW recognition domain of Pin1, apparently in a mode that does not inhibit PPIase activity.
To enforce complexation into the larger active site rather than into the tighter WW domain of Pin1 and to enhance the overall binding affinity, we designed a perhydropyrrolizine scaffold substituted with additional aromatic residues (Fig.,5).
A novel, straightforward synthesis towards this class of compounds was developed (Schemes,4 and 5), and the racemic compounds (±)- 22a,22d were found to inhibit Pin1 with Ki values (Ki,=,inhibition constant) in the micromolar range (Table,2).
To further enhance the potency of these inhibitors, the optically pure ligands (+)- 22a and (+)- 33b,c were prepared (Schemes,6 and 7) and shown to inhibit Pin1 with Ki values down to the single-digit micromolar range.
According to 15N,1H-HSQC-NMR spectroscopy and enzymatic activity assays, binding occurs at both the WW domain and the active site of Pin1.
Furthermore, the new synthetic protocol towards perhydropyrrolizines was extended to the preparation of highly substituted perhydroindolizine ((±)- 43; Scheme,8) and pyrrolidine ((±)- 48a,b; Scheme,9) derivatives, illustrating a new, potentially general access to these highly substituted heterocycles.
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Urea bonded cyclodextrin derivatives onto silica for chiral HPLC

JOURNAL OF SEPARATION SCIENCE, JSS, Issue 12 2006I. Wayan Muderawan
Abstract
Several structurally well-defined perfunctionalised cyclodextrin chiral stationary phases (CD CSPs) for high performance liquid chromatography have been successfully prepared by immobilisation of perfunctionalised cyclodextrins on silica through urea linkage(s) using the Staudinger reaction.
These CSPs show high chiral recognition efficiency and are utilised in the resolution of various types of racemic compounds.
This paper reviews the development of sixteen perfunctionalised cyclodextrin-based CSPs, their preparation, and their application to enantioseparation of seventy-seven racemic compounds under a range of separation conditions.
[source]

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 6 2000T. Ito
ABSTRACT
Photophysical characteristics of N-substituted C5,C5,-linked dihydrothymine dimers (1a,b[meso], meso compounds of [5R,5,S]-bi-5,6-dihydrothymines; 1a,b[rac], racemic compounds of [5R,5,R]-bi-5,6-dihydrothymines and [5S,5,S]-bi-5,6-dihydrothymines) in aqueous solution with varying contents of less-polar aprotic solvent such as tetrahydrofuran or dioxane have been investigated by UV-absorption, and steady-state and time-resolved fluorescence spectroscopies.
Among the C5,C5,-linked dimers, (5R,5,S)-bi-5,6-dihydro-1-methylthymine (1a[meso]) showed a redshifted weak UV-absorption band at 270,350 nm and excimer fluorescence emission at ,max= 370 nm with a quantum yield (,F) of ,0.1 in phosphate buffer (pH < 10) at 293 K. Racemic compound of 5,6-dihydro-1-methylthymine dimer (1a[rac]), meso and racemic compounds of 5,6-dihydro-1,3-dimethylthymine dimers (1b[meso] and 1b[rac]) in phosphate buffer were nonfluorescent under similar conditions.
The UV-absorption and fluorescence spectral characteristics of 1a[meso] in aqueous solution were interpreted in terms of intramolecular stacking interactions between the dihydropyrimidine chromophores leading to a preferential "closed-shell" conformation both in the ground state and the excited singlet state.
In basic solutions at pH > pKa= 11.7, the fluorescence quantum yield of 1a[meso] decreased due to a dominant "open-shell" conformation resulting from the electrostatic repulsion between the deprotonated dihydrothymine chromophores of 1a[meso] in a dianion form.
[source]

A list of organic kryptoracemates

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2010László Fábián
A list of 181 organic kryptoracemates has been compiled.
This class of crystallographic oddities is made up of racemic compounds (i.e. pairs of resolvable enantiomers) that happen to crystallize in Sohnke space groups (i.e. groups that include only proper symmetry operations).
Most (151) of the 181 structures could have crystallized as ordered structures in non-Sohnke groups.
The remaining 30 structures do not fully meet this criterion but would have been classified as kryptoracemates by previous authors.
Examples were found and checked with the aid of available software for searching the Cambridge Structural Database, for generating and comparing InChI strings, and for validating crystal structures.
The pairs of enantiomers in the true kryptoracemates usually have very similar conformations; often the match is near-perfect.
There is a pseudosymmetric relationship of the enantiomers in about 60% of the kryptoracemate structures, but the deviations from inversion or glide symmetry are usually quite easy to spot.
Kryptoracemates were found to account for 0.1% of all organic structures containing either a racemic compound, a meso molecule, or some other achiral molecule.
The centroid of a pair of enantiomers is more likely (99.9% versus 99% probability) to be located on an inversion center than is the centroid of a potentially centrosymmetric molecule.
[source]

Potential of different techniques of preferential crystallization for enantioseparation of racemic compound forming systems